1. Field of the Invention
This invention relates to microwave rotating waveguide joints that transmit microwave energy from a stationary source to feed a mechanically steerable microwave transmitter, and more particularly to a universal microwave waveguide joint that allows for simultaneous 3-axis rotation and 3-dimensional translation between the transmitter's antenna and the stationary source.
2. Description of the Related Art
A mechanically steerable microwave transmitter includes a source that generates a beam of microwave radiation, an antenna that projects the beam, through foe space, and a gimbal mechanism, that rotates the antenna about the Azimuth and Elevation axes to point the beam in any direction of a hemisphere and a waveguide to direct the beam from the output of the stationary source to the antenna feed. Exemplary sources may include a magnetron or klystron that produce a high power beam of microwave radiation having a frequency within approximately 100 MHz to approximately 300 GHz, roughly spanning the L-band to the G-band. Exemplary antennas may include a slotted waveguide array, reflector or horn. The antenna, may be either uni-directional in which it only transmits the beam or bi-directional in which it may either transmit or receive microwave radiation.
Two important problems in the design of a gimbaled transmitter are coupling the beam from the source to the antenna and minimizing the beam loss between the source and the antenna. In one straightforward approach, the source is affixed to the antenna and must be supported and moved by fee gimbal mechanism. This approach is not desirable for many transmitter systems due to the weight and bulk of the source, which, in turn requires that the gimbaling mechanism be larger and heavier than desirable.
Responsive to this problem, transmitter systems have been developed wherein the source is stationary, and a waveguide extends from the source to the antenna. As used herein a “waveguide” is hollow conductive pipe. Waveguides are typically rectangular or circular and formed from metal The width of the waveguide is typically on the order of the wavelength of the transmitted microwave beam. For example, a circular waveguide supports different TEmn or TMmn modes of beam propagation where “m” and “n” refer to the number of sinusoidal half cycles the field pattern makes in the circumferential “m” and the radial “a” directions. The TE11 mode is known as the “dominant” mode in a circular waveguide and the TE01 mode is a “non-dominant” mode. The waveguide has one or more rotary joints to allow the antenna to rotate with respect to the stationary source. Each rotary joint allows for 1-axis of motion, i.e. roll about the axis through the rotary joint. One rotary joint is mounted on the elevation gimbal support and another rotary joint is mounted on the azimuth gimbal support.
An exemplary microwave rotary joint is illustrated in U.S. Pat. No. 7,973,613. The rotary joint includes a pair of circular waveguides one of which is fixed and one of which rotates inside the other. A pair of mode converters is connected to the circular waveguides at the input and output, respectively, of the rotary joint. One of the mode converters converts a rectangular TE10 mode from the source to the axial symmetric circular TE01 mode that propagates through the rotary joint. The other mode converter converts the TE01 mode back to a rectangular TE10 mode to feed the antenna.
Because the rotary joints are mounted on the gimbal mechanism, one each on the elevation gimbal support and the azimuth gimbal support, the center of mass of the antenna is shifted away from the azimuth and elevation axes. Typically heavy and bulky counter weights are added to shift the center of mass back to the azimuth and elevation axes to mass-balance the assembly. This further increases the total mass that must be driven to steer the antenna.